Self-threading screw

ABSTRACT

Self-tapping screw with rolled thread turns, on which the angle bisector of the thread turn formed by the load flank and the rear flank slopes away from the screw head at an angle to the perpendicular to the screw axis, the load flank running in a straight line to the thread root, and the rear flank initially extending in a straight line from the thread tip and, at about ⅓ of its height, merging via a kink into a straight angled portion at an angle that is greater than the angle, measured relative to the perpendicular, of the rear flank and is between 30° and 50°, the ratio of the thread&#39;s core diameter to the thread&#39;s outside diameter being greater than 0.7 and the ratio of the thread pitch to the thread&#39;s outside diameter being less than 0.25.

[0001] The invention relates to a self-tapping screw with rolled threadturns, on which the angle bisector of the thread turn formed by the loadflank and the rear flank slopes away from the screw head at an angle tothe perpendicular to the screw axis.

[0002] A screw of this kind is known from European Patent Application501 519. The special feature of this known screw is that both the loadflank (the thread flank facing the screw head) and the rear flank (theflank opposite to the load flank) of its thread turns run into thethread root via rounded portions. This rounded configuration ispresented as being advantageous in the introduction to the publication,in contrast to DE-A 32 35 352 cited therein. In the case of thesubject-matter of this DE-A 32 35 352, the transition between the flankprofile and the core region of the thread is of discontinuous design,this transition consisting namely of a kink. In contrast, DE-A 32 07975, which is also cited in the European patent application, furthermorediscloses a self-tapping screw where the flank angle of the thread turnsincreases continuously from the flank tip to the thread root, this beingintended to promote damage-free deformation of the plastic in which thescrew concerned is to be screwed. In addition, the European patentapplication points out in relation to the screw from DE-A 32 07 975that, in the case of the screw described there, which is intended to bescrewed into plastic, the thread turns are not asymmetrical, i.e. theangle bisector between the load flank and the rear flank of the threadturns is perpendicular to the screw axis.

[0003] A self-tapping screw created for use preferably with particleboard is furthermore known from European Patent Application 713 017, inwhich the cross section of the thread turns has a kink in the region ofthe load flank, such that the load flank runs at a larger angle relativeto the perpendicular to the screw axis after the kink from the directionof the thread tip. Moreover, in the case of this screw, the anglebisector between the load flank and the rear flank slopes towards thescrew head in the region between the thread tip and the kink. As aresult, the thread turns of this screw form a bearing surface that isclose to the perpendicular when tightened or subjected to load, thethread turn bearing particularly heavily against material in therelevant component as the said material presses against it. As aconsequence, the individual thread turns are subjected to bending stressby forces that run virtually parallel to the thread axis, considerablyreducing the loading capacity of this screw.

[0004] Self-tapping screws disclosed in European Patent Application133773 and DE-A 36 15 271 have a similar configuration since, with theseknown screws, the forces acting on the thread turns when a load isimposed are virtually parallel to the screw axis in the region betweenthe thread tip and a kink in the load flank. This is particularly markedin the case of the screw disclosed in DE-A 36 15 271, in which the loadflank runs perpendicular to the screw axis in the region between thethread tip and the kink. The two known screws are thus particularlysensitive when being tightened and hence to high bending loads on thethread turns.

[0005] The object on which the invention is based is to configure thescrew described at the outset in such a way as to improve its productionby thread rolling and furthermore to increase its load-bearing capacityand holding strength. At the same time, the geometric configuration ofthe screw is to be chosen in such a way as to ensure optimum screw-inperformance in the respective metallic material.

[0006] According to the invention, this is accomplished by the fact thatthe load flank runs in a straight line to the thread root, and the rearflank initially extends in a straight line from the thread tip and, atabout ⅓ of its height, merges via a kink into a straight angled portionat an angle that is greater than the angle, measured relative to theperpendicular, of the rear flank and is between 30° and 50°, the ratioof the thread's core diameter to the thread's outside diameter beinggreater than 0.7 and the ratio of the thread pitch to the thread'soutside diameter being less than 0.25.

[0007] By virtue of the essentially rectilinear profile from the threadtips to the thread root in the region of the load flank and of the kinkin the rear flank, the screw can be rolled with rolling dies whosegrooves each have a corresponding rectilinear profile in cross section,a considerable advantage over rolling dies that are rounded in this areawhen it comes to the production of the rolling dies. Moreover, it hasbeen found that the dimensioning that results when the ratio of thethread's core diameter to the thread's outside diameter is greater than0.7 leads to a shortening of the thread turns, with the result thatthese are subjected to bending loads over only a relatively small radialheight when the screw is tightened and when subjected to loading by thecomponent concerned, these loads furthermore being taken in an effectivemanner by virtue of the fact that the angled portion extends over theregion that requires firm support against buckling for the thread turns.At the same time, the fact that the kink starts at only about ⅓ of theheight of the thread turns means that there is sufficient space at theoutside of the thread turns to allow the displaced material from thecomponent concerned to flow away, and this process of flowing away istherefore not impaired.

[0008] Owing to the continuously straight sloping profile of the loadflank of the thread turns, the force that is exerted on this flank whenthe screw is tightened is introduced at a correspondingly oblique angleinto the screw and hence its core because of the slope of the load flankand, because of its relatively large diameter, the core is capable ofabsorbing large forces. Conversely, the pressure in the material of thecomponent is likewise directed at an oblique angle, i.e. into thematerial surrounding the location at which the screw is screwed in,where this pressure can be readily absorbed. At the same time, therectilinear slope of the load flank enables uniform pressuredistribution over the entire height of the thread's load flank, therebyalways allowing a maximum area of contact and maximum friction duringtightening of the screw without damaging the material. By virtue of theratio of the thread pitch to the thread's outside diameter, which ischosen to be less than 0.25, the thread obtained has closely spacedthread turns, with the result that a relatively large number of threadturns is anchored in the component concerned over a given length. Thisresults in correspondingly high tear-out and holding forces.

[0009] The angle between the angle bisector (between the load flank andthe rear flank) and the perpendicular to the screw axis isadvantageously chosen in such a way as to be between 5° and 15°. Theresulting slope of the thread turns leads to an adequate slope of theload flank, even in the case of small flank angles (measured between theload flank and the rear flank), with the result that forces acting onthe load flank are introduced into the core of the screw in a favourablemanner. The slope and the straight profile of the load flank are thusresponsible for the friction between the screw screwed in and thecomponent, which must be sufficiently large to ensure that theoverturning torque, at which the screw would strip the thread in thecomponent, is not reached too easily.

[0010] The screw according to the invention is particularly suitable forscrewing into steel, for which purpose the screw itself is naturallylikewise composed of steel. In this case, a flank angle between the loadflank and the rear flank of 38° to 48° has proven advantageous sincethis gives an optimum relationship between the displaced volume andstress-bearing capacity of the material. If the steel screw according tothe invention is screwed into light alloy, a flank angle between theload flank and the rear flank of 32° to 42° is expedient in this case.

[0011] It is also possible to make the screw from aluminium, inparticular a hard aluminium alloy, which is then likewise suitable forscrewing into light alloy. In this case, the selected flank anglebetween the load flank and the rear flank is expediently between 58° and68°.

[0012] The slope of the angle bisector is particularly favourable if itis chosen to be about ⅙ of the flank angle between the load flank andthe rear flank.

[0013] Exemplary embodiments of the invention are illustrated in thefigures, in which:

[0014]FIG. 1 shows a screw with a screw head and a self-tapping thread,

[0015]FIG. 2 shows a section through part of the thread in FIG. 1 inenlarged representation,

[0016]FIG. 3 shows a section through the thread similar to that in FIG.2 with a flank angle of 45°,

[0017]FIG. 4 shows a section at a similar angle through a thread with aflank angle of 33°,

[0018]FIG. 5 shows a section through a thread with a flank angle of 60°.

[0019]FIG. 1 shows a self-tapping screw with a screw head 1 and the core2, into which the thread 3 has been rolled. The thread concerned has asingle start with the individual thread turns 4, which slope away fromthe screw head 1 at an angle to the perpendicular 5 to the screw axis 6.A screw-driving means 7 has been stamped into the screw head 1 and is ofknown configuration.

[0020] In FIG. 2, the thread 3 from FIG. 1 is shown in section inenlarged representation. The individual thread turns 4 have the loadflank 8, which faces towards the screw head 1, and the opposite rearflank 9. The load flank 8 extends in a straight line from the thread tip10 to the thread root 11. The rear flank 9 opposite to the load flank 8merges via the kink 12 into the angled portion 13. The load flank 8 withits flank angle γ and the rear flank 9 with its flank angle δ, each inrelation to the perpendicular 5, form the flank angle β, which in thiscase is 39°. The angle bisector 15 of the flank angle β, whichessentially determines the slope of the thread turns 4, forms the angles relative to the perpendicular 5. In relation to the perpendicular 5 tothe screw axis 6, the angled portion 13 runs at an angle α, which islarger than the rear flank angle δ, measured between the perpendicular 5and the rear flank 9.

[0021] Also entered in FIG. 2 are the pitch P (distance between twoadjacent thread turns 4), the outside diameter D of the thread and thecore diameter K of the thread. In the case of the thread illustrated inFIG. 2, the ratio of the thread's core diameter K to the thread'soutside diameter D is 0.8, while the ratio of the thread pitch P to thethread's outside diameter D is 0.17. In this case, the flank angle β is39°.

[0022] FIGS. 3 to 5 show variants of the thread in FIG. 2. In the caseof the thread shown in FIG. 3, the flank angle β is 45°, in the case ofthe thread shown in FIG. 4, the flank angle β is 33° and, in the case ofthe thread shown in FIG. 5, the flank angle β is 60°.

[0023] As regards the configuration of the threads shown in FIGS. 2 to4, it should furthermore be noted that the thread root 11 is in eachcase cylindrical.

[0024] It should furthermore be noted that the exemplary embodiments ofthreads shown in enlarged representation in FIGS. 2 to 5 representranges of favourable flank angles β that can be used for particularapplications, as explained in detail above. In the case of the threadshown in FIGS. 2 to 5, the kink 12 in the region of the rear flank 9 isat ⅓ of the height of the thread turns 4, this height being measuredfrom the thread root 11 in the direction of the thread tip 10. In thecase of the thread shown in FIGS. 2 to 5, the angled portion 13 startingfrom the kink 12 runs at an angle α (see FIG. 2), which is 45°.

1. Self-tapping screw with rolled thread turns (4), on which the anglebisector (15) of the thread turn (4) formed by the load flank (8) andthe rear flank (9) slopes away from the screw head (1) at an angle tothe perpendicular (5) to the screw axis (6), characterized in that theload flank (8) runs in a lo straight line to the thread root ( 11), andthe rear flank (9) initially extends in a straight line from the threadtip (10) and, at about ⅓ of its height, merges via a kink (12) into astraight angled portion (13) at an angle (α) that is greater than theangle (δ), measured relative to the perpendicular (5), of the rear flank(9) and is between 30° and 50°, the ratio of the thread's core diameter(K) to the thread's outside diameter (D) being greater than 0.7 and theratio of the thread pitch (P) to the thread's outside diameter (D) beingless than 0.25.
 2. Screw according to claim 1, characterized in that theslope (ε) of the angle bisectors (15) is in a range of between 5° and15°.
 3. Screw according to claim 2, characterized in that the slope (ε)of the angle bisectors (15) is about ⅙ of the flank angle (β) betweenthe load flank (8) and the rear flank (9).
 4. Steel screw according toone of claims 1 to 3 for screwing into steel, characterized in that theflank angle (β) measured between the load flank (8) and the rear flank(9) is 38°-48°.
 5. Steel screw according to one of claims 1 to 3, forscrewing into light alloy, characterized in that the flank angle (β)measured between the load flank (8) and the rear flank (9) is 32°-42°.6. Aluminium screw according to one of claims 1 to 3 for screwing intolight alloy, characterized in that the flank angle (β) measured betweenthe load flank (8) and the rear flank (9) is 58°-68°.